The systems are being installed in San Francisco, Baltimore, and other cities with funding from the Department of Homeland Security, according to the Daily, which obtained copies of contracts, procurement requests, specs and other documents.

The use of the equipment raises serious questions about eavesdropping without a warrant, particularly since recordings of passengers could be obtained and used by law enforcement agencies, says Wired.

The IP audio-video systems can be accessed remotely via a built-in web server and can be combined with GPS data to track the movement of buses and passengers throughout the city.

Audio and video can be monitored in real-time, but are also stored onboard in blackbox-like devices, generally for 30 days, for later retrieval. Four to six cameras with mics are generally installed throughout a bus, including one near the driver and one on the exterior of the bus.

Cities that have installed the systems or have taken steps to procure them include San Francisco, California; Eugene, Oregon; Traverse City, Michigan; Columbus, Ohio; Baltimore Maryland; Hartford, Connecticut; and Athens, Georgia.

Here’s a quick overview of the power of the spectrogram on Audacity, the free, open source software for recording and editing sounds.

Spectrographic software, combined with array mikes, are commonly used to track animals.

PMEL’s Ishmael acoustic monitoring software tracks whales by their acoustic footprint. Spectrograms in Ishmael show time on the horizontal axis and frequency on the vertical axis. A call is “detected” when the sound template exceeds a threshold match for a set length of time.

A spectrogram application for the iPhone uses the iPhone’s built-in microphone to create realtime scrolling spectrograms of any sound you’re hearing, letting you see spectrograms in the field, at the very same time that you’re listening to the bird sound.